FIG. 6 is a perspective view illustrating a semiconductor laser device using a conventional can-type hermetically sealed package. In FIG. 6, a semiconductor laser chip 104 is mounted on a heat sink 105 via a submount 106. The heat sink 105 with the laser chip 104 and a monitor photodiode 107 ante mounted on a base plate (eyelet part) 112 comprising iron or the like. Those elements on the base plate 112 are covered with a cap 100. The cap 100 comprises a cap body 10.1 and a window 116. The cap body 101 is made of Kovar alloy (an alloy containing Fe, Co, Ni, or the like) and has an opening at its top surface. The diameter of the opening is 3.about.4 mm. The window 116 is adhered to the cap body 101 using Pb containing glass 117, such as PbSn glass, covering the opening of the cap body 101. Reference numeral 108 designates a lead for the monitor photodiode 107, numeral 109 designates a lead for grounding, and numeral 110 is a lead for the laser chip 104. An Au wire 111 connects the laser chip 104 to the lead 110. An Au wire 118 connects the photodiode chip 107 to the lead 108. Generally, a semiconductor laser device used for optical communication has an oscillation wavelength of 0.98 .mu.m, 1.2 .mu.m, 1.3 .mu.m, or 1.55 .mu.m. For example, a laser chip including an InGaAs/GaAs multiquantum well structure is used for a laser device having an oscillation wavelength of 0.98 .mu.m, and a laser chip comprising InGaAsP series materials is used for a laser device having an oscillation wavelength of 1.2 .mu.m, 1.3 .mu.m, or 1.55 .mu.m. In this prior art device, the cap 100 is welded to the base plate 112, so that the package of the semiconductor laser device 200 is hermetically sealed.
FIGS. 7(a) and 7(b) are diagrams illustrating process steps in a method of fabricating the cap 100 of the semiconductor laser device 200. In these figures, the same reference numerals as in FIG. 6 designate the same or corresponding parts. Reference numeral 101a designates an opening of the cap body 101.
Initially, as illustrated in FIG. 7(a), a circular window 116, a cap body 101, and an annular glass plate 117 are prepared. The circular window 116 is formed by polishing or coating a glass plate used for optical parts and cutting the glass plate into a circular plate having a diameter of 3.about.4 mm. The cap body 101 has a circular opening whose diameter is smaller than the diameter of the window 116. The annular plate 117 comprises Pb containing glass and has an inside diameter larger than the diameter of the opening 101a and an outside diameter smaller than the diameter of the window 116.
Thereafter, as illustrated in FIG. 7(b), the annular glass plate 117 is put on the inner surface of the cap body 101 along the periphery of the opening 101a, and the window 116 is put on the annular glass plate 117. Then, the structure of FIG. 7(b) is heated in a furnace to melt the annular glass plate 117, whereby the window 116 is adhered to the cap body 101, producing the cap 100.
Thereafter, the cap 100 is fixed to the base plate 112 on which the laser chip 104 and the photodiode chip 107 are mounted, preferably by electric welding, resulting in the semiconductor laser device 200 shown in FIG. 6.
In operation, laser light emitted from the laser chip 104 is output through the window 116 and utilized as a desirable light source of a device utilizing the laser light.
In the above-described semiconductor laser device, since the window 116 is adhered to the cap body 101 using the Pb containing glass 117, when the Pb containing glass 117 can melt due to external influences, e.g. humidity, and contaminate the window 116, so that the characteristics of the laser device are degraded and the airtightness of the package is reduced.
Further, since the window 116 comprises glass, not only laser light but also visible light, i.e., light having a wavelength of 400.about.750 nm, is transmitted through the window 116. Therefore, the inside of the cap 100 can be visually observed through the window 116 after fabrication of the laser device. In this case, if the elements in the package look abnormal due to the arrangement of the elements, the device is judged as defective even though the performance of the device is normal, whereby the production yield is reduced.
Furthermore, since the window 116 comprises glass, the processing of the glass plate, such as polishing, coating, and cutting, is difficult, so that the production cost of the window 116 increases, resulting in an expensive cap 100.
Meanwhile, ELECTRON MATERIAL 21 (February 1983, p. 60) discloses an infrared sensor employing silicon as a window material that selectively transmits light having a prescribed wavelength, utilizing the fact that silicon does not transmit light having a wavelength shorter than 0.9 .mu.m. In a light responsive optical semiconductor device having the above-described structure, a part of incident light having a desired wavelength is selected by and transmitted through the silicon window, so that the light responsive element responds to the light of the desired wavelength. In addition, the element is not adversely affected by an unwanted part of the incident light. Therefore, the performance of the optical semiconductor device is improved. However, since the silicon window and the cap body are connected with an adhesive, when the adhesive partially melts due to external influences, especially humidity, the window may be contaminated and the airtightness of the device reduced.